352 GENETICS IN RELATION TO AGRICULTURE 
have been reported. While many first generation crosses between 
different species are more vigorous than either parent, others are known 
to be exceedingly weak. Unless repeating crosses which have already 
been made, the hybridizer of species is exploring the unknown and there 
is always the possibility that his results may be of interest to science as 
well as of practical value. : 
The Svalof Method of Creating Populations.—Progress in plant im- 
provement by means of hybridization experiments will always be limited 
by the available supply of experts as well as by facilities and time. Any 
method, therefore, that enables the breeder to secure desirable new 
combinations of parental characters without the enormous amount 
of detail involved in a system of pedigree culture, is worthy of serious 
consideration. Such a method was devised by Nilsson-Ehle and has been 
used at Svaléf with success. According to Newman, “‘two known sorts 
are crossed and the whole progeny from all second and succeeding gen- 
erations is sown together en masse. The object of this plan is to allow 
the severe conditions of winter and early spring to either destroy or 
expose the weaknesses of as many of the more delicate combinations as 
possible. In the latter case the breeder is given an opportunity to assist 
nature in her work of elimination by practising a form of mass-selection. 
While there is thus effected in a very simple manner a gradual weeding 
out of a great mass of unfit combinations, the progeny of a crossing at 
the same time gradually assumes the character of an ordinary mixed 
population, the different combinations becoming automatically constant 
as time passes. The advantages of working with constant forms will be 
appreciated by all breeders as will also the fact that through the above 
arrangement the number of combinations which may arise through the 
repeated segregation of inconstant forms in each succeeding generation 
will have increased immensely. . . . While the above system requires 
a considerable length of time before any definite results can be reached, 
yet it requires very little work until the time comes to make selections. 
Numerous crossings of this kind may therefore be carried forward with 
the regular work and thus provide a constant source of new material.” 
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